Control of stability through overlap matching: closo-carborynes and closo-silaborynes

Abstract

The matching of ring and cap orbitals for overlap is used to arrive at the best carborynes among the many possibilities. Accordingly, 1,2-carboranes, 1,2-silaboranes (C₂B_nH_n+2, and Si₂B_nH_n+2, n = 4, 5, 8, and 10), and their dehydrogeno derivatives were studied with use of the Density Functional Theory (B3LYP/6-311+G^∗). The dehydrogenation of 2,3-C₂B₅H₇ (6a) to 2,3-C₂B₅H₅ (13a) is estimated to be even less endothermic than those of benzene and 1,2- C₂B₁₀H₁₂ (1a) to benzyne and 1,2-C₂B₁₀H₁₀ (8a) by more than 21 kcal/mol. This is due to the extra stabilization gained through better overlap of the C₂B₃H₃ ring with the 2 BH caps. The relatively larger size of the Si atom leads to overlap requirements in silaboranes that are different from those in carboranes. The lower Si-Si single bond energy and the preference of Si for lower coordination result in unusual structures in dehydrogenosilaboranes. One of the Si atoms moves away from the surface in Si₂B₁₀H₁₀ (15), Si₂B₈H₈ (16, 17, and 18), and 1,2-Si₂B₅H₅ (19). One Si atom forms a bridge to a trigonal surface in 2,3-Si₂B₅H₅ (20) and 1,2-Si₂B₄H₄ (21). Estimates of three-dimensional aromaticity with NICS calculations show that the exohedral double bond does not influence three-dimensional aromaticity.